Desorption spectra

B. Meng, W. H. Weinberg. Monte Carlo simulation of temperature programmed desorption spectra. J Chem Phys 700 5280-1589, 1994. 

A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values Tx, Ts,. . ., Tn for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra s(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd is amenable as well as a better resolution of superimposed peaks on a desorption curve (see Section VI). What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. 

Some experimental desorption spectra can be fitted with the calculated curves only if the assumption of constant values of Ed and kd for all peaks is abandoned, and Ed and/or kd are considered to be functions of the coverage. 

A quantitative treatment based on the following approach has been recently given to the idea of explaining the multiplicity of desorption spectra by the existence of different desorption mechanisms rather than by different adsorption states (98, 117). Consider a surface on which an adsorption equilibrium has been established at a given temperature. On heating the surface, desorption occurs, the probability of which is composed of at... 

In all probability, further attempts at elucidating the physical background of the phenomenon of multiple desorption spectra will appear in the near future. 

Figure 19. Thermal desorption spectra of water adsorbed on (I) Ag(l 10), (2) PK111), (3) Ru(001), and (4) Ni(UO). (Reproduced from P.A. Thiel and T.E Madey, Surf. Sci. Reports 7 258, Fig. 29,1987, Ci 1987 with permission of Elsevier Science.)...

Thermal desorption spectra, 171 Thermodynamic equilibrium, phase transitions at, 219 Thermodynamic phase formation, passivation potential and, 218 Time resolved measurements in the microwave frequency range, 447 photo electrodes and 493 Tin... 

Figure 2.2. Thermal desorption spectra of carbon monoxide, measured mass spectrometically at mass 28 (atomic units, a.u.), on a platinum (100) surface upon which potassium has been pre-adsorbed to a surface coverage of 0K.7 Reprinted with permission from Elsevier Science.

Figure 2.33. Thermal desorption spectra of oxygen from mixed oxygen-chlorine adlayers on Pt(100).9S The initial chlorine and oxygen concentrations as well as the dosing temperatures are indicated in the figure. Heating rate 20 K s 1.95 Reprinted with permission from Elsevier Science.

Figure 4.43. Thermal desorption spectra after gaseous oxygen adsorption on a Pt film deposited on YSZ at 673 K and an 02 pressure of 4x 10"6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (I=+15 pA) for various time periods.29-30 Reprinted from ref. 30 with permission from Academic Press.

Figure 4.45. Thermal desorption spectra (bottom) and corresponding catalyst potential variation (top) after electrochemical O2 supply to Ag/YSZ at 260-320°C at various initial potentials Uwr Each curve corresponds to different adsorption temperature and current, thus different values of Uwr, in order to achieve nearly constant initial oxygen coverage.31 Reprinted with permission from Academic Press.

Figure 5.3. Oxygen thermal desorption spectra after electrochemical O2 supply to Pt/YSZ at 673 K (I = +12 pA for 1800 s) followed by isothermal desorption at the same temperature at various times as indicated on each curve.4,7 Reprinted from ref. 7 with permission from Academic Press.

Figure 5.21. Experimental setup (inset) showing the location of the working (WE), counter (CE) and reference (RE) electrodes and of the heating element (HE) thermal desorption spectra after gaseous oxygen dosing at 673 K and an 02 pressure of 4x1 O 6 Torr on Pt deposited on YSZ for various exposure times. Oxygen exposure is expressed in kilo-langmuirs (1 kL=l0 3 Torrs). Desorption was performed with linear heating rate, ()=1 K/s.4 S Reprinted with permission from Academic Press.

Thermal desorption spectra of CO2 from a titania surface are shown in figure 2. It revealed two desorption peaks at temperature ca. 175 and 200 K. As reported, surface of titania have two structures which is similar to the results fomd by Tracy et al. [7]. Based on their study, it was confirmed that one peak at ca. 170 K was attributed to CO2 molecules bound to regular five-coordinate Ti site considered as the perfected titania structure. The second peak at ca. 200 K considered as the CO2 molecules bound to Ti referred to the... 

Fig. 2. Thermal desorption spectra for CO2 Adsorbed on titania samples...

The signal is relatively constant in the laser desorption spectra as expected. 

The existence of various temperature intervals characterized by predominant manifestation of one of above interactions can be detected from thermal desorption spectra. For instance, the thermal desorption spectrum obtained in [71] for a cleaved ZnO (1010) monocrystal following its interaction with oxygen (Fig. 1.4) indicates the availability of such typical temperature intervals as interval of physical adsorption (a), chemisorption (b), interval of formation of surface defects (c) and, finally, the domain of formation of volume defects (d). 

Habenschaden, E. and Kiippers, J. (1984) Evaluation of flash desorption spectra , Surf. Sci., 138, L147. 

Thermal desorption spectra of carbon monoxide on polycrystalline and on single crystal platinum are well known from experiments in the gas phase [48,49], The system is therefore appropriate to test the experimental setup. 

Fig. 2.3. Thermal desorption spectra after adsorption from the gas phase (a) adsorbed CO on Pt (b) H2 on Pt.

Fig. 2.4. (a) Thermal desorption blank experiment. The Pt electrode was held at 0.45 V vs. RHE in the base electrolyte (5 x 10 2 M H2S04) during 120 s and then transferred to the UHV. (b) Thermal desorption spectra of adsorbed CO on Pt after adsorption from an aqueous solution. Temperature scan 5 K/s. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

To demonstrate the kind of information that TDS reveals and to explain how TDS spectra are analyzed, we use the set of desorption spectra of Ag atoms from an... 

Figure 2.11 Thermal desorption spectra of silver from the close-packed surface of ruthenium for different initial Ag coverages. Desorption from the second layer of silver occurs at lower temperatures, indicating that Ag-Ag bonds are weaker than Ag-Ru bonds. Note the exponential increase of the low temperature sides of the peaks, indicating that the desorption follows zero-order kinetics (from Niemantsverdriet et al. [18]).

Figure 9.14 Thermal desorption spectra of CO from clean (left) and potassium-promoted Ni (110) (middle and right) mea-sured at a heating rate of 13 K/s. The spectra exhibit two desorption states for CO on promoted surfaces and indicate that CO binds more strongly to sites adjacent to potassium (from Whitman and Desorption Temperature (K) Ho [46]).

Fig. 5. H2 (2 amu) and D2 (4 amu) thermal desorption spectra from chemisorbed and on Pt(lll), respectively.

Temperature Dependence of Secondary Ions. We now consider the relationship between SIMS spectra and desorbable hydrogen and hydrocarbon species in more detail by comparing the temperature dependence of the various hydrocarbon containing ions and RU2C2 with the thermal desorption spectra of Figure 1. 

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